Industry in general has been actively seeking a novel approach to capture and store waste heat efficiently such that it can be utilized at a more opportune time. Further, the desire to achieve energy storage in a compact space demands the development of novel materials that are capable of storing high energy content per unit weight and unit volume. Areas of potential application of breakthrough technology include transportation, solar energy, industrial manufacturing processes as well as municipal and/or commercial building heating.
In the transportation industry, exhaust aftertreatment systems, also known as emission control devices, are used to reduce pollutant emissions. Such aftertreatment systems typically remove pollutants from exhaust gases after they are discharged from the combustion chamber. They include, as examples, catalytic converters, diesel particulate filters, and diesel oxidation catalysts.
It is known in some applications that a solution of urea is injected into the exhaust gas stream of the vehicle to aid in the reduction of NOx (i.e., nitrogen oxide), such as nitrogen II oxide (i.e., NO) and/or nitrogen IV oxide (i.e., NO2), in a vehicle's emission control system. In the emission control system, (e.g., in an ammonia producing reactor of an emission control system) the urea solution is converted into ammonia (NH3) and CO2. The ammonia reacts with nitrogen oxides NOx contained in the exhaust gas in the SCR reactor and thus converts harmful NOx into benign reaction products: nitrogen gas (i.e., N2) and water (i.e., H2O). There are however some disadvantages using the urea solution in this application. For example, commercial aqueous solution of urea (e.g. AdBlue™, 32.5 wt % urea) has almost 7 times more water than is needed for stoichiometric hydrolysis of urea into NH3 and CO2. The injection of urea solution is typically carried out by spraying into an ammonia producing reactor, before entering, along with exhaust gas, the SCR reactor. Commercial aqueous solution of urea (e.g. 32.5 wt. % urea/67.5 wt. % water solution meeting ISO 2224 requirements and designated as AdBlue™ by the German Association of the Automobile Industry, such as Fleetguard® Diesel Exhaust Fluid (DEF) available commercially from Cummins Filtration, and BlueTEC® available commercially from Daimler AG) has almost 7 times more water than is needed for stoichiometric hydrolysis of urea into NH3 and CO2. If this solution is injected directly into the ammonia producing reactor, the excess water will cause cooling of the exhaust gas, as the excess water consumes heat (e.g., latent heat of vaporization and sensible heat). This may result in a reduction in temperature such that the ammonia producing reactor and or the SCR reactor does not function efficiently, especially in situations when the temperature where the urea solution is sprayed is relatively low (e.g., about 300° C. or less, or about 250° C. or less). The low temperature usually happens when the exhaust temperature and flow rate are low in urban driving conditions and/or when the vehicle is stopped with its engine idling. When this happens, it can result in a solid deposit formation and/or suboptimal temperature in the SCR reactor. This cooling effect can also result in higher fuel consumption due to high degree of exhaust gas recirculation (EGR) needed to keep the amount of emitted NOx low, since EGR remains the only NOx emission reducing means when the SCR reaction is too slow due to low temperatures in the SCR reactor. In addition, to reduce NOx emission, urea needs to decompose to release ammonia (NH3) so that chemical reactions can be efficiently carried out in the SCR reactor. The processes of evaporation of excess water in the urea solution and the decomposition of urea to produce ammonia are both endothermic.
The use of urea solution has other disadvantages. The fuel efficiency of a vehicle is being compromised due to the extra and unnecessary water weight in the urea solution a vehicle has to carry. Some commercially available urea solutions freeze at −11° C. When that happens, steps will have to be taken to melt the urea solution. For example, a vehicle's exhaust system may operate with suboptimal, minimal or even no reduction of NOx, emissions until the engine compartment warms up from the heat of combustion and melts the urea solution.
An alternative to the use of urea solution in reducing NOx in a vehicle's exhaust aftertreatment system was disclosed in US Patent Publication No. 2008/0260597, incorporated herein by reference in its entirety. This patent publication discloses a solid reductant rod pressed against a heating element as a means to produce ammonia for SCR on demand. While this may solve the problems presented by extra water of urea solutions, the invention disclosed in this patent publication requires heat generation near the solid reductant in the solid reductant reactor in order to decompose the reductant to generate reducing gas. The heat generation requirement adds a parasitic load for the engine and the alternator, which in turn reduces the fuel efficiency of the vehicle.
Despite the benefits of using a solid reductant, in some vehicle applications, an aqueous solution (containing about 32.5 wt. % urea) is the preferred means of providing the reductant, e.g., due to handling, dosing, and delivery benefits of a liquid.
There is a need to heat both solid and liquid (e.g., aqueous solutions) reducing materials in order to efficiently reduce emissions of NOx, particularly when the temperature of the exhaust gas is generally low. As such, there is a need for an alternate source of heat so that a sufficient temperature of the exhaust gas is maintained for efficient reduction of NOx. For example, there is a need for devices, systems and processes for providing heat to a vehicle exhaust system that does not add a parasitic load to the engine and/or alternator for generating the heat. There is also a need for devices, systems, processes, and materials for reducing NOx emissions that function efficiently at low temperatures (e.g., about −15° C. or less).